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Explanation of ENUM (RFC 2916). Patrik F ältström Area Director, Applications Area , IETF. Problem we want to solve. ENUM. 2. Problem statements. How do network elements (gateways, SIP servers etc) find services on the Internet if you only have a telephone (E.164) number?

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Explanation of enum rfc 2916

Explanation of ENUM (RFC 2916)

Patrik Fältström

Area Director, Applications Area, IETF

Problem statements
Problem statements

  • How do network elements (gateways, SIP servers etc) find services on the Internet if you only have a telephone (E.164) number?

  • How can subscribers define their preferences for incoming communications?

Today many addresses
Today, many addresses





With enum only one
With ENUM, only one







Give this number to friends:


Solution in short
Solution in short

  • Put domain names derived from telephone numbers in the global domain name system, DNS

  • IETF ENUM Working Group created to solve the problem of using the DNS for:

    • domain name in

    • [Numbers re-formatted as domain names]

    • URI out

    • [mailto, sip, tel, http or other URI scheme]

  • Solution: NAPTR records

  • Use the URI’s for the communication

Enum in a nutshell
ENUM in a nutshell

  • take phone number


  • turn into domain name

  • ask the DNS


  • return list of URI’s


Enum use in more detail



Sip server

ENUM use in more detail




“Call setup”














Calling party

Called party

Voip via sip to voip


VoIP via SIP to VoIP






Voip via pstn to pstn








Step 1

The input to the

NAPTR algorithm

Step 1

  • Take an E.164 number and create a fully qualified domain name in a single highly defined and structured domain

  • +46-8-971234

  • +468971234


Step 1 explanation
Step 1, Explanation

  • Each digit can become a definable and distributed “zone” in DNS terms

  • Delegation can (doesn’t have to) happen at every digit, including at last digit

  • Zones such as country codes, area codes or primary delegated blocks of numbers can be delegated as well as individual numbers

  • DNS defines authoritative name servers for NAPTR/service resource records

Step 2
Step 2

  • Lookup NAPTR RR’s in DNS, and apply NAPTR/ENUM algorithm



  • Use rewrite rules using regular expressions which operate on the E.164 number (+468971234)

Regular expressions
Regular Expressions

  • For ENUM, the NAPTR regexp field may yield an (unchanged) URL

  • !<regexp>!<string>!

    • “Match <regexp> on original E.164, and apply rewrite rule <string>”

^ - Match beginning

$ - Match end

. - Match any character

.* - Match any number of any character

() - Grouping, \n in <string> is replaced with group number ‘n’ in <regexp>

Step 2 in detail
Step 2 in detail


  • IN NAPTR 10 10 ”U” ”mailto+E2U” ”!^.*$!mailto:spam@paf.se!”

  • IN NAPTR 20 10 ”U” ”ldap+E2U” ”!^+46(.*)$!ldap://ldap.telco.se/cn=0\1”

  • Note that no line break should be in the records

Status done
Status (done)

  • RFC 2916 published in September 2000

  • The domain e164.arpa delegated to RIPE NCC

    • RIPE NCC appointed by the IAB according to RFC 2916

  • ITU SG2/WP1/2 liaison statement:

    • Member state can inform ITU on entry of numbers in ENUM tree

Status done1
Status (done)

  • Instructions to RIPE NCC

    • Any “request” not coming from TSB is to be forwarded to TSB

    • If TSB says no, no delegation is to be done

    • All evaluation of any “request” is to be done by TSB

    • RIPE NCC is only creating and evaluating the technical data which they need for the actual delegation

    • RIPE NCC is to communicate with TSB

    • RIPE NCC is to make public any communication regarding a “request”

Status not done
Status (not done)

  • Delegation of domains below e164.arpa to managing entities that are appointed by member states

    • A few “requests” have been passed to TSB, which have responded very quickly

  • National issues are not resolved

    • We have not heard that issues related to verification of holder of E.164 number are resolved anywhere

Issue 1 competition
Issue 1: Competition

  • The DNS requires a unique root

    • More on this later

  • In DNS, we have a question on how to have competition in general, since one commercial company “had too large a market share”

  • Competition for registrations of DNS records is ensured using a registry/registrar model:

    • One registry which runs the DNS server

      • Competition at this level is handled when choosing the registry, because that can only be one organization

    • Many registrars which the customer talk with

      • Registrars decide for example price for a registration

Issue 2 1 unique root
Issue 2.1: Unique root

  • Why is a unique root used in ENUM?

    • Say I have phone number +468971234

    • I can choose between a.com and b.com

    • I choose b.com, and get the DNS domain


    • Axel is to call me

    • He knows my phone number, +468971234, only

    • How is he to know that it is in b.com and not a.com I have my data?

  • General issues in RFC 2836

  • We need a “Golden Tree” for E.164 numbers

Issue 2 2 selection of root
Issue 2.2: Selection of root

  • Address and Routing Parameters Area

  • See contribution from ISOC

  • See http://www.iana.org/arpa-dom/

    • in-addr.arpa

      • IPv4-addresses

    • ip6.arpa

      • IPv6-addresses

    • e164.arpa

      • E.164 numbers

Issue 3 alternatives
Issue 3: Alternatives?

  • The implementation of a globally unique ENUM tree provides ample opportunity for competition at the national level

  • Having ENUM using domain e164.arpa means that one still have:

    • Ability to have dialing plans in other domains

    • Competition regarding registration according to registry/registrar model on all levels in DNS tree

    • Competition when selecting the registries

    • Competition regarding services

3 1 alternative dial plans
3.1 Alternative Dial plans

  • A user very seldom dial an E.164 number on his phone

    • Inside Cisco, people dial 58509 to me

    • Outside Cisco, from Sweden, people must dial 0014085258509

  • Not all dialed numbers are E.164 numbers

  • People use dialing plans

  • Dialing plan information can be resolved by the use of same technology as ENUM

  • 3 2 registry registrar
    3.2 Registry / Registrar

    • A registry runs the DNS server for a specific domain name

      • one domain -> one registry

    • A customer contacts a registrar when he want to have things registered, not the registry

    • Registrar verify customer data, do billing, and send data to registry when delegation is to be done, changed or cancelled

      See next slide…

    3 3 selection of registry
    3.3 Selection of registry

    • Registries are needed which runs the DNS

      • On a country code level

      • Inside a country code, according to local policy

    • The model we use today with DNS is

    Holder of a domain





    Holder of a domain


    3 4 services
    3.4 Services

    • A customer in ENUM announces what services he subscribes to

      • Examples:

        • Email

        • Web homepage

        • SIP service (VoIP)

        • Telephony

    • Each one of these services can be handled by separate contracts between customer and service provider

    • These contracts have nothing to do with the announcement via ENUM of their existence

    The big picture
    The big picture…




    Registry for country

    Registrar for country

    Registrar for country

    Registrar for country

    ENUM Service provider

    ENUM Service provider

    Service provider

    for email

    ENUM Service provider

    Service provider

    for email

    Service provider

    for email

    Service provider

    for telephony

    Service provider

    for telephony

    Service provider

    for telephony


    Explanation of enum rfc 2916

    Patrik Fältström